1. Field of the Invention
The present invention concerns optical fiber couplers employed in communication systems.
2. Prior Art
In one type of optical fiber coupler known in the art, two or more fibers are aligned side by side in a plane and thermally fused and elongated, thereby forming a fused-elongated region.
In the fused-elongated region thus formed, the diameter of each component optical fiber is reduced, as is the diameter of the core of each fiber. To the extent that diameter of the cores of the optical fibers are reduced, a proportionately larger fraction of the light propagated therein leaks through the cladding which surrounds the core of each fiber. Also, to the extent that the component optical fibers are drawn out and thereby elongated, the distance between the cores of adjacent optical fibers is reduced, and due to this fact, the coupling between the propagated modes of the individual fibers becomes extremely great. In this way, the light signal carried by one fiber branches and is thus caused to be multiplexed over two or more optical fibers.
However, the above described optical fiber coupling technique has the following shortcomings:
1. When conventional quartz glass single mode optical fiber is used to form the optical fiber coupler, because the properties of the material are such that very little light energy is lost through the lateral surface of the fiber, the fused-elongated region must be quite long in order to sufficiently achieve optical coupling. Further, in order to form a fused-elongated region of sufficient length, the resulting diameters of the component optical fibers in the fused-elongated region end up being considerably reduced. For example, starting with quartz optical fibers having an outer diameter of 125 .mu.m, it is possible that in the fused-elongated region of the resulting coupler, the fibers have a final outer diameter of on the order of 20 .mu.m. Because of this effect, even a relatively small curvature in the optical fiber results in light leaking through the side of the fiber to the exterior with loss in light energy. With a larger amount of bending in the optical fiber, the bending losses become significant.
2. When forming the fused-elongated region, if it is desired to limit the amount of elongation and still achieve the desired degree of optical coupling, the length of the fused region must be considerable. Thus even though the elongation of the optical fiber has been limited in such a case, the coupler ends up being quite long. Accordingly, in answer to applications calling for miniaturization, such an optical fiber coupler cannot be practically employed.